How can I fill a cylinder cap?
Triangles

As @ideasman42 points out in this answer, you can press:


  F, &#9095; AltP
  
  
  F (creates a face)
  &#9095; AltP (poke face, creating a triangle fan)
  


Quads

As @David said in this answer, you can use grid fill by:


Using &#9095; Alt +  Right Click, to select the edges
Pressing Ctrl + F and then selecting Grid Fill (can be done by pressing G after the previous combination)